1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus, such as a color copying machine, a color printer or the like. More particularly, the present invention relates to a power transmitting device for transmitting power to each of a plurality of developing devices disposed on an outer circumference of a photosensitive body for developing an electrostatic latent image formed on the photosensitive body to form an image, and an image forming apparatus with the same.
2. Description of the Related Art
In general, an electrophotographic type color image forming apparatus such as a color copying machine, a color printer or the like comprises a plurality of developing devices which develop an electrostatic latent image formed on the photosensitive body to form a toner image. Each developing device is provided with a developing roller for developing an electrostatic latent image corresponding to a predetermined color, for example, one of yellow, magenta, cyan and black.
The developing roller of each developing device is spaced from the photosensitive body by a substantially constant gap, and power is transmitted to the developing roller by a power transmitting device during rotation of the photosensitive body to develop an electrostatic latent image when the developing process is carried out.
FIG. 1 is a view of a conventional power transmitting device 1 for transmitting power to each developing roller of a plurality of developing devices.
This power transmitting device 1 in the developing device comprises a plurality of driving sections 51, a plurality of coupling sections 57, and a rotational shaft 40.
Each driving section 51 comprises a plurality of rotational members 52. A driving gear is mounted to an outer circumferential surface of each rotational member 52, and the driving gear of each rotational member is coupled to a driving motor (not shown) through a gear train of the developing device. Each rotational member 52 has a push cap 53 formed at an upper end, and the push cap is elastically coupled to a frame (not shown) by a support spring (not shown) disposed around the push cap.
Each coupling section 57 comprises a female coupling 58 formed at a lower surface of each rotational member 52 and a male coupling 59 formed on an end portion of a shaft 6 of each developing roller. The female coupling 58 is a coupling recess, and the male coupling 59 is a coupling protrusion having a shape corresponding to the coupling recess of the female coupling 58.
The rotational shaft 40 is coupled with a cam gear 10 through a one-way spring clutch 20 that transmits power in only one direction. The cam gear is coupled to a power restricting motor 27 through a power restricting gear train (not shown). A plurality of cams 42 are provided on an outer circumferential surface of the rotational shaft 40 at regular intervals.
Accordingly, when the rotational shaft 40 is rotated at a certain angle by the power of the power restricting motor transmitted through the cam gear 10, one cam 42 provided on the rotational shaft 40 pushes the push cap 53 of the corresponding rotational member 52 in the direction of the arrow “C” against an elastic force of the support spring. As a result, the female coupling 58 of the rotational member 52 is coupled with the corresponding male coupling 59 of the shaft of the developing roller, and a rotational force of the rotational member 52 rotated by the driving gear coupled with the driving motor of the developing device through the power transmission gear train is transmitted to the shaft 6 of the developing roller.
As shown in FIG. 2A and FIG. 2B, however, each cam 42 provided on the rotational shaft 40 has symmetrical cam surfaces 43. Therefore, normal lines NL1′ and NL2′ (that is, lines which are normal to the surface of the cam) formed at the points at which the cam begins to contact or begins to disengage the push cap 53 of the corresponding rotational member 52, intersect a line CL′ connecting a rotational axis O′ and a maximum stroke point MP′ of the cam 42.
Accordingly, when the push cap 53 of each rotational member 52 is pushed in the direction of the arrow “C” by the corresponding cam 42, as shown in FIG. 2A, reaction forces F1′ and F2′ applied to the cam surface 43 by the push cap 53 generate a rotational torque in the opposite direction (arrow B) of the rotational direction (arrow A) of the cam 42, which is rotated by the spring clutch 20 at a start point IP′ at which the female coupling 58 begins to couple with the male coupling 59 by the cam surface 43. Also, as shown in FIG. 2B, the reaction forces generate a rotational torque in the direction which is same as the rotational direction (arrow A) of the cam 42 at a finish point LP′ at which the female coupling 58 begins to disengage from the male coupling 59 by the cam surface 43.
As shown in FIG. 2B, when the cam surface 43 is halted around the finish point LP′ due to an excessive rotation of the cam, the cam 42 receives the rotational torque generated by the reaction force F2′ in its rotational direction (arrow A), and causes a clutch spring (not shown) of the spring clutch 20 to be unwound. As a result, the clutch spring is loosened, and the rotational shaft 40 is unstable at the halted position, and may be further rotated in the direction of the arrow A.
As described above, when the rotational shaft 40 is further rotated in the direction of the arrow A and then halts, the rotational member 52 contacts the cam 42 via the push cap 53 and is moved in the direction of the arrow D by an elastic force of the support spring. As a result, the female coupling 58 of the rotational member 52 is unstable when it is disengaged from the male coupling 59 formed on the shaft 6 of the developing roller. Accordingly, the transmission of the rotational force of the rotational member 52 to the shaft 6 of the developing roller is unstable, and it is possible that no force is transmitted to the shaft 6 of the developing roller. Thus, the power transmitting device in the developing device as described above cause inferior electrostatic latent images on the photosensitive drum, and may result cause poor development of the electrostatic latent images.
Accordingly, there is a need for an improved power transmitting device for a developing device in an image forming apparatus.